circle concepts

Angle c is right because if a side on a triangle inside the circle(inscribed triangle) is the diameter the triangle is always a right triangle. 

Some examples of circle concepts:

The straight green lines outside of the circle are tangent lines because they touch exactly one point on the circle, the faint line connecting points of tangency is a chord which is a line connecting any two points in the circle without crossing the diameter.  

angle a and angle b are congruent because they catch the same arc which is the pink part of the circle.

Angle c,b and a are all central angles because they are all in the center of the angle.

Angle a is the inscribed angle which is catches one point anywhere on the circumference of the circle.

line pba is secant because it is a chord that extends outside the cirlce.